This application proposes to extend the Women's Health Study (WHS) for an additional 3 years of randomized treatment and follow-up. The WHS is an ongoing randomized, double-blind, placebo-controlled, 2x2 factorial trial evaluating the balance of benefits and risks of low-dose aspirin and vitamin E in the primary prevention of cardiovascular disease and cancer among 39,876 U.S. female health professionals aged 45 and older. Its goal is to provide either clear positive results or definitive null results on which sound clinical and public health recommendations for the use of low-dose aspirin and vitamin E can be made. Based on a review of the unblinded data, both the clinical and methodologic members of the trial's Data and Safety Monitoring Board unanimously recommended an extension of both randomized components beyond the trial's scheduled termination in August 2001. The primary rationale for the Board's recommendation was the likelihood of ambiguity at the scheduled end of the trial regarding the findings of both agents on both endpoints, ambiguity that could result in considerable clinical confusion. Three additional years will help clarify these ambiguities, more conclusively determine the balance of benefits and risks of these agents for the primary prevention of cardiovascular disease, and allow an adequate latent period to elapse for assessing the effects of both agents on cancer, thus providing clear clinical recommendations for health care providers and patients. The Women's Health Study is in a unique position to answer these important questions. Average duration of treatment is 6 years; morbidity and mortality follow-up rates are 98 percent and 100 percent, respectively. Compliance with both agents at 72 months is 75.6 percent. Pilot data indicate that 97 percent of compliers would definitely be willing to continue the trial for 3 additional years or would consider continuing. This trial is cost effective, at 75 dollars per randomized participant per year in direct costs. Given the gaps in knowledge this study is intended to address, as well as the certain intense interest in its findings by the medical, lay, and regulatory communities, the proposed extension of this trial in women is timely and important.